Regulation of mRNA stability plays a critical role in the control of gene expression. Dysregulated mRNA turnover has been implicated in various pathological processes. Understanding the regulation of mRNA decay should provide insights into the pathogenesis of certain disease. An important cis-acting element responsible for rapid mRNA decay in mammalian cells is the AU-rich element (ARE), which directs mRNA decay through a process referred to as ARE-mediated mRNA decay (AMD). We have investigated the function of KSRP, a decay-promoting ARE-binding protein, in AMD for the past seven years. To further understand the regulation of KSRP activity in AMD and identify it's in vivo physiological mRNA targets, we have initiated a purification scheme to isolate proteins that associate with KSRP and have generated Ksrp- null mice. This grant renewal will utilize a series of biochemical and molecular approaches to achieve the following specific aims. Specific aim 1 is to characterize function of DDX1, an RNA helicase that was found to co-purify with KSRP, in regulation of AMD and the subcellular localization of KSRP. This aim will also purify KSRP complexes in a large scale, identify proteins that co-purify with KSRP by mass spectrometry, and investigate their roles in the regulation of AMD. Specific aim 2 is to investigate the roles of Ksrp in post- transcriptional control of type I interferon (IFN-1 and IFN-2) gene expression and in virus infection using Ksrp knockout mice. This aim will examine whether Ksrp regulates the decay of Ifna and Ifnb mRNAs and the effect of Ksrp deficiency on virus infection. Specific aim 3 is to identify Ksrp target mRNAs in LPS- stimulated macrophages. We will identify mRNAs that are stabilized in LPS-stimulated Ksrp-/- macrophages by genome-wide analysis of mRNA decay and those associated with Ksrp by ribonucleoprotein immunoprecipitation and microarray analysis. Our long-term goals are to understand the mechanism by which KSRP regulates AMD, to identify it's in vivo mRNA targets, and to characterize phenotypes associated with the dysregulation of the targets. The generation of Ksrp knockout mice should provide a valuable tool to study the post-transcriptional regulation of certain cytokine mRNAs by Ksrp, which should contribute to our understanding of the pathogenesis of inflammatory and immunological diseases resulting from a defect in mRNA decay.